The conventional auto carburetor is constructed as shown diagrammatically in FIG. 1. In such a carburator the fuel is first introduced through a fuel supply pipe f into a float chamber a, where the fuel level is kept constant by means of the float b, acting on the float valve c. The fuel level and the level of the fuel jet port d at the center of the Venturi V in the intake conduit are at substantially the same level. A negative pressure is developed in the Venturi V by air being sucked therethrough into the engine and this negative pressure draws the fuel from the float chamber a into the Venturi V through the fuel jet port d through the throttle e.
Designating the pressure upstream of the Venturi V as P.sub.1, the velocity of the air upstream as V.sub.1 (V.sub.1 =0), the pressure of the fluid in the Venturi V as P.sub.2, the velocity of the fluid in the Venturi V as V.sub.2 and the specific gravity of air as .rho.a, the following relationships exist according to Bernoulii's law: ##EQU1## Therefore, the negative pressure in the Venturi V, being equal to ##EQU2## may be considered to be the velocity head.
Meanwhile, a pressure difference occurs between the pressure upstream and the pressure downstream of the throttle e. If the flow velocity in the throttle e is V.sub.3, and the height of the fuel level in the float chamber a above the horizontal part of the throttle e is h and the specific qravity of fuel is .rho.t, the pressure in the throttle e can be expressed, also according to Bernoulli, as follows: ##EQU3## where g is the acceleration due to gravity and C is a flow constant. Thus the negative pressure in the throttle e which is equal to ##EQU4## is constant. The reason is that, if ##EQU5## the value ##EQU6## which is a constant, so that the air/fuel ratio can be maintained constant regardless of how the volume of air sucked in may change. As a matter of fact, however, even if the air/fuel ratio is set appropriately for a given air volume, it will be different for a different air volume. The reason is that, in the conventional system described above, the air flow in the Venturi V is turbulent, whereas the fuel flow in the throttle e is laminar, which the above equation (a) does not take into account.
The present invention seeks to overcome this drawback in conventional carburators.